The present application describes a multilayered sheet, specifically a multilayered sheet that may be both thermoformable and thermally stable.
Packaging film and packaging sheet are used for many purposes. One of these many purposes includes thermoforming the film or sheet into packages, such as trays, plates or other containers, which may then be used to package products. Another of these many purposes includes packaging products subjected to thermal processing, including but not limited to retort, aseptic, hot-fill or microwave processing.
One packaging sheet that is currently used for thermoforming into packages is disclosed in U.S. Patent Application Publication Pub. No. US 2011/0104342 (published May 5, 2011). This sheet comprises a first rigid component and a second rigid component and has a normalized combined tear initiation and propagation resistance in both machine direction and transverse direction of less than about 0.115 in*lbf/mil (0.0511 Ncm/micron) energy to break and an elongation in both machine direction and transverse direction of less than about 0.800%/mil (0.0315%/micron) as measured in accordance with ASTM D1004. The first rigid component and the second rigid component are exemplified by identical blends of high impact polystyrene (HIPS) and general purpose polystyrene (GPPS). Thermoformed packages formed from this sheet having a first rigid component having an analogous composition as a second rigid component are expected to distort and deform when exposed to temperatures above 194° F. (about 90° C.) due to the presence of the polystyrene. One sheet that is currently used for thermoformed packages exposed to temperatures above 194° F. (about 90° C.) comprises a sheet having two exterior layers of polypropylene. However, a sheet with two exterior layers of polypropylene is expected to be difficult to thermoform. Additionally, the methods of manufacturing a sheet with two layers of polypropylene are expected to be limited due to polypropylene's crystallinity and other properties. What is needed is a sheet that may be both thermoformable and thermally stable.